In the 1970s, scholars in social psychology began exploring the process by which individuals attach their own identity to the groups in which they associate. This gave rise to social identity theory, which rests on the notion that, through largely unconscious cognitive processes, individuals who value and closely identify with a particular social group (e.g., familial, ethnic, religious, gender, partisan, national, etc.) will tend to take on characteristics and exhibit behaviors that are consistent with positive attributes associated with that group. Social identity theory also suggests that individuals do more than merely identify with the social groups to which they belong; they also derive comfort, security, and self-esteem from these groups. As a result, group members often engage in favoritism toward their own social group and, at times, denigration of other social groups as a way to protect or enhance their own group identity. Because individuals identify with multiple groups, the concept of salience is also crucial to our understanding of social identity theory. Specifically, individuals will seek to protect or enhance a particular group identity (through words or actions) when they perceive it to be threatened or they sense an opportunity to promote or enhance it. Given the obvious import and relevance of these dynamics to various aspects of society, research on social identity theory has grown exponentially over the past several decades, especially within the social sciences. Scholars in the fields of psychology, sociology, political science, and communication, for example, have increasingly paid attention to and incorporated social identity theory into their study of everything from how politicians communicate to how people vote to how people interact with other cultures. Notably, within the field of communication, the value of social identity theory rests with its ability to explain or predict messaging and response behaviors when a particular group identity is made salient. Thus, social identity theory is a robust theoretical framework that, in recent years, has had broad appeal and application across a number of academic disciplines. With a focus on the intersection of social identity theory and communication research, this article seeks to identify the foundational works within this area of research, recognize the primary journals in which this research can be found, discuss the key concepts and terms associated with this research, and explore how social identity theory has evolved both theoretically and empirically since its inception in the 1970s.